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Sunday, 21 July 2019

Properties of Conducting Materials

PROPERTIES OF CONDUCTING MATERIALS:


1. Conductivity (σ) :

The conductivity (σ) is the reciprocal of electrical resistivity of the material. The units of conductivity are mhos/cm. It is the property of a material due to which the electric current flows easily through the material. In other words it provides an easy path to the flow of electric current through the material.

2. Tensile Strength :

Strength of a material is defined as the ability to resist load without failure. Tensile strength is therefore tie ability of the material resist a stretching (tensile) load without fracture. Therefore the tensile strength gives an indication of the conductor limit within which it has to be used and beyond which excessive deformation or fracture takes place. It is expressed in load per unit cross sectional area. (tonnes / cm2).

3. Ductility :

It is the ability of the material to be deformed plastically without rupture under tensile load. A ductile material can be drawn out into a fine wire without fracture and can also be bent, twisted or changed in shape without fracture. Gold, silver, copper, aluminium, nickel, tin, lead etc., are ductile materials.

4. Corrosion Resistance :

Corrosion is a gradual process in a material due to electro-chemical attack. Due to the chemicals present in the atmosphere and if the material is exposed, the metal is generally converted into an oxide, salt or some other compound, thus the metal- does not serve the purpose it is intended to. It may also occur in elevated temperature in media which are inert when near or below the room temperature.



5. Effect of Alloying on Resistivity :

Any impurity whether metallic or non-metallic increases resistivity. The effect of metal impurities on resistivity of a given metal is dependent on the nature of alloy formed. When the metal differs widely in atomic volumes and melting points the alloy comprises of crystals of both metals. Such alloys are called mechanical mixtures. The resistivity and temperature co-efficient of resistivity in these alloys vary linearly with percentage content of impurity.

When the atomic volumes of both do not differ by more than 15% the alloy comprises of single crystal in structure in which the crystal lattice accommodates the atoms of both metals. This is called solid solution alloy. In this type of alloy, the resistivity increases and temperature co-efficient decreases upon certain percentage of impurity content. After this, if the impurity is increased the resistivity decreases and temperature co-efficient increases.

When the metals combine chemically, it is called chemically combined alloy. The variation of resistivity and temperature co-efficient of resistivity with percentage of impurity content is very complicated. The alloying will increase the mechanical strength to a considerable extent and the material will become hard.

6. Effect of Alloying on Mechanical Properties :

Addition of even a small percentage of certain alloys improves mechanical and physical properties. Some of the effects are summarized below:

(a) Copper : It increase the strength and hardness and lowers the ductility. It also increases the corrosion resistance.

(b) Aluminium: It acts as de-oxider and restricts grain growth and aids nitriding.

(c) Lead : It increases machinability.

(d) Nickel : It increases tensile strength without sacrificing ductility. Increases toughness lowers the co-efficient of thermal expansion. Increases hardness slightly and decreases rusting.

(e) Tungsten : Carbide forming tendency high inhibits grain growth and considerably increases cohesive force.

(f) Silicon : Improves magnetic permeability and electrical resistivity. Increases the resistance to oxidization. Raises the ultimate strength. Acts as a ferrite strengthener.

(g) Chromium : Increases strength and hardness without affecting ductility. Increases wear and corrosion resistance. Increases the critical temperature and improves toughness.

(h) Manganese : Lowers melting point of iron decreases critical temperature increases the hardenability of steel.

(i) Cobalt : Imparts excellent magnetic properties, increases hardness in high speed steel.

7. Solderability :

Solder is a fusible alloy used to join the surfaces of metals. The property is useful at places where the two pieces of metals are to be joined as in the case of wires.

8. Brittleness:

It is opposite to the property of toughness and is the tendency of a metal to break on receiving a hammer blow. The brittle material has a poor resistance to shock loads.

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